The present invention relates to a process for monitoring the state of a wheel of a motor vehicle.
The key problem in monitoring the state of a wheel of a motor vehicle consists of determining the rotational speed. According to the prior art, a starting point for determining the rotational speed of a wheel of a motor vehicle is the use of the existing hardware from wheel slip control systems, such as an antilock system ABS, an anti-slip control with traction control ASC+T and others. In known systems, a sensor acts preferably inductively with a transducer wheel that is connected rigidly to the wheel. In automobiles the transducer wheel usually has 48 almost equidistant teeth for generating an electric output signal, which is evaluated as a measure of an instantaneous rotational speed of the wheel. By doing so, when there is a loss of pressure in the wheel, there is an increase in the rotational speed because of the decrease in the wheel diameter. To this end, the output signal of the sensor is converted into an essentially rectangular pulse signal. At the same time the respective duration of the individual pulses is measured.
In this respect the most accurate knowledge possible of the rotational speed of the motor vehicle wheel is less interesting for determining the speed of the motor vehicle itself than for monitoring the wheel tire. For example, modern tires have a limp home property, which permits driving up to a speed of approx. 80 km/h even with severe loss of pressure in the tire, without the vehicle driver having to be absolutely aware of a defective tire directly from the changed handling characteristics of the vehicle. However, beyond the specified threshold the vehicle is no longer operating safely. Thus, for safety reasons the driver must be cautioned by a monitoring system about a defect of a tire. For this and other reasons modern vehicles are increasingly equipped with tire control and tire pressure monitoring systems, whose function is based on the most accurate knowledge possible of the rotational speed of each of the four motor vehicle wheels or on the use of pressure sensors.
German Patent DE 197 35 313 A1 describes a process for determining the rotational speed used to evaluate a higher frequency content in the wheel speed signal for the purpose of identifying a loss of pressure in the tire. With this process, following the measurement of the duration of a pulse, an especially weighted average between a current value of the duration and a preceding value is determined. To measure the duration of the pulses of the speed sensor, the time between the two upward slopes of a rectangular signal is measured by means of a pulse width measurement, by counting the pulses of a clearly higher frequency signal, and is stored tentatively as the count of the counter t1. The counts t1 are read out at fixed times T1 and made available to a further development environment. If within a computing cycle, several counts are measured, then a mean value is formed. Its reciprocal value is an absolute measure for the rotational speed of the wheel. The higher the count of the counter, the lower the rotational speed. In this manner the width of each tooth of the pulse wheel is measured by means of a high frequency pulsing oscillation crystal and is stored tentatively in a buffer. At equidistant times T1, corresponding to a computing cycle of the processing algorithm, the buffer is read out and a mean value is formed from the counts that have arrived. Thus, a current value and a preceding value lie a specific step size apart and/or they exhibit a specific time interval. In one embodiment the comparison values are symmetrical to a value that is to be currently evaluated. This embodiment is described with reference to a figure in the drawings. Additionally a correction factor comprising each deviation between the duration of a current value and a respective mean value is determined. The rigid coupling of the transducer wheel with the tire and a synchronization between the evaluating algorithm and the angular state of the vehicle wheel results in a fixed allocation between a respective tooth of the transducer wheel and a correction factor. Recurring errors in the system are to be eliminated by means of the respective correction factors. In this respect, reference is made to the content of German Patent DE 197 35 313 A1 for the details of this known process and embodiments.
The object of the present invention is to improve the above described process for monitoring the state of a wheel while improving the reliability and accuracy and to provide a corresponding device.
It is known from the above described process of the prior art that in a process for monitoring the state of a wheel of a motor vehicle, an electrical output signal of a sensor is converted into an essentially rectangular pulse signal. At the same time a respective duration of the pulses is measured; and an especially weighted average between the duration of a current value and a preceding value is determined. In so doing, a current value and a preceding value exhibit a time interval or they are spaced a specific step size apart. A correction factor including each deviation between the duration of a current value and a respective mean value is determined.
In contrast, a process according to the invention is characterized by matching a step size of the correction method to an error order of a fault in order to increase the reliability of the corrected measurement results. In this respect the present invention is based on the recognition that faults can be classified according to orders. In practice the faults usually includes partial faults of different orders, whereas in a base model of the present invention for classifying a fault, a mathematical Fourier analysis of a real physical interference signal is performed. Thus, a maximum cycle duration of systematic faults of a wheel tire is given by the simple circumferential length of the tire and is reproduced by a fixed allocation to the teeth of the pulse transducer wheel. Thus, a fault of first order is described; in reality it is an eccentricity error or an axial offset, without fundamental oscillation. Errors of second or higher order represent the harmonics of this fundamental oscillation. The effect of the adaptation of a step width on a respective fault and/or fault order is explained in detail below with reference to the figures of the drawings.
In a further development a process, according to the invention, exhibits a step size j, which can be determined from a periodic deviation (to be eliminated) of the mth order, according to the following formula:j=1/m×n/4.
With respect to the number of pulse teeth on a pulse transducer wheel, of size n, a distinction is made between the motor vehicle area with n=48 and an application in motorcycles with n=96 teeth. For the motor vehicle area as a preferred field of application of the invention the specified formula for an embodiment is thus simplified to:j=1/m×12.
Preferably at least two different step sizes j are applied in combination. The result is, in particular, a correction method as a multi-step process, which is carried out preferably as a series connection of two methods of the invention. For motor vehicles the step sizes j=12 and j=3 are used. The reason for this choice is explained in turn by means of the description of an embodiment and with reference to the figures of the drawings. At this stage the surprising dominance of specific orders of fault and the relatively few means for their elimination shall be discussed in detail here. The same applies to motorcycles, hence, there is no need for a separate discussion here.
In a significant further development of the invention, filtering takes place to reduce the influence of external disturbing stimuli. During each revolution of the wheel, a newly calculated current factor ki−j is filtered with an event-triggered filter according to a PT1 structure in order to obtain, as the result, an improved correction factor ki. In a preferred embodiment of the invention, the filter calculates at an instantaneous revolution N for the respective tooth with index I, a weighted average kN.i. between the instantaneous, just calculated factor ki and the averaged correction factor kN−1.i of the previous revolutions. It carries out the following transfer function:kN.i=(a×kN−1.j+k1)/(a+1)
In the conversion of a process, according to the invention, the goal is to obtain definitely improved values for determining the rotational speed. Said improved values result in reliable values for determining a reference state, i.e. with respect to its internal pressure. The correction factors, determined for correcting the drifted sensor signals, can be used advantageously together with their fixed allocation to a respective tooth for determining a nonuniformity of a wheel. Building on the described process, the determined correction factors can also be evaluated in order to detect a change in the wheel uniformity and thus, as in the known process for loss of pressure in the tire, to record this change and/or to trigger a warning with or without further intervention. For a detailed description of the process and the definite advantages over the prior art process with respect to accuracy and reliability, reference is made at this point to the drawing and description of an embodiment.
A unit, constructed according to the present invention, can also be adapted in a simple manner to an application in smaller or larger vehicles using varying signal sources or sensors and still maintain the aforementioned advantages. Thus, it can also be adapted especially to motorcycles with a pulse transducer wheel with n=96 teeth. Thus, with the use of an inventive process and a corresponding device, a unit, according to the invention, can be adapted to another area of application. Thus, using the computing capacity of already existing microprocessors of prior art, braking and/or traction control systems, a wheel can be monitored in time slots of approximately 5 ms duration, which have not been used to date in past systems. Owing to a significantly improved evaluation of the speed with plausibility check, one embodiment of the invention enables in particular a more reliable and optimized estimate of the pressure; in addition, a nonuniformity and even a change in the nonuniformity in relation to the wheel can be determined. In processing according to a process of the invention, the speed signal for the wheel slip control systems exhibits a significantly improved quality over the prior art.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.